Butanol is an important industrial chemical, with a variety of applications, such as use as a fuel additive, as a feedstock chemical in the plastics industry, and as a foodgrade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this chemical will likely increase.
Several chemical synthetic methods are known; however, these methods of producing butanol use starting materials derived from petrochemicals and are generally expensive and are not environmentally friendly. Several methods of producing butanol by fermentation are also known, for example the ABE process which is the fermentive process producing a mixture of acetone, 1-butanol and ethanol. Acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum is one of the oldest known industrial fermentations; as is also the pathways and genes responsible for the production of these solvents. Production of 1-butanol by the ABE process is limited by the toxic effect of the 1-butanol on Clostridium acetobutylicum. In situ extractive fermentation methods using specific organic extractants which are nontoxic to the bacterium have been reported to enhance the production of 1-butanol by fermentation using Clostridium acetobutylicum (Roffler et al., Biotechnol. Bioeng. 31:135-143, 1988; Roffler et al., Bioprocess Engineering 2:1-12, 1987; and Evans et al., Appl. Environ. Microbiol. 54:1662-1667, 1988).
In contrast to the native Clostridium acetobutylicum described above, recombinant microbial production hosts expressing 1-butanol, 2-butanol, and isobutanol biosynthetic pathways have also been described. These recombinant hosts have the potential of producing butanol in higher yields compared to the ABE process because they do not produce byproducts such as acetone and ethanol. However, the problem with these recombinant hosts is that biological production of butanol appears to be limited by butanol toxicity thresholds to the host microorganism used in the fermentation. Extractive fermentation methods have not been applied to the production of butanols using recombinant microbial strains.
The present invention satisfies the above need and provides a method of making butanol from at least one fermentable carbon source that overcome the issues of toxicity resulting in an increase in the effective titer, the effective rate, and the effective yield of butanol production by fermentation utilizing a recombinant microbial host wherein the butanol is extracted into specific organic extractants during fermentation.